Pixel Mask File (pixmask3.fits)
The pixel mask file consists of a single FITS format image file for each observation blocked by 1 for ACIS observations and blocked by 2 for HRC observations in SKY coordinates. The pixel values are either '1' or '0', where '1' indicates that any detections that occupy this pixel will be included in the CSC 2.0, whereas detections that fall on those pixels with a '0' are excluded.
In general, all of the ACIS-I array chips are used, but the edges of the ACIS-S array chips are excluded. Areas that are significantly affected by read-out streaks are also excluded. This means that the pixel-mask is more-restrictive (has a smaller area) than the corresponding field-of-view file (fov3), as shown in the example below.
The pixel mask FITS file is named: 〈i〉〈s〉〈obs〉_〈obi〉N〈v〉[_〈c〉]_pixmask3.fits
where 〈i〉 is the instrument designation, 〈s〉 is the data source, 〈obs〉 is the observation identification, 〈obi〉 is the observation interval identification, 〈v〉 is the data product version number, and 〈c〉 is the cycle. The optional discriminator identified in square brackets is included only for ACIS alternating exposure (interleaved) mode observations.
Example
An example pixmask3 file from release 2.0 is:
unix% dmlist acisf00637_001N020_pixmask3.fits.gz blocks
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Dataset: acisf00637_001N020_pixmask3.fits.gz
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Block Name Type Dimensions
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Block 1: mask Image Byte(3901x3334)
The image contains a standard Chandra set of coordinate mappings (to both the sky and celestial systems):
unix% dmlist acisf00637_001N020_pixmask3.fits.gz cols
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Columns for Image Block mask
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ColNo Name Unit Type Range
1 mask[3901,3334] Byte(3901x3334)
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Physical Axis Transforms for Image Block mask
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Group# Axis#
1 1,2 sky(x) = (+2681.50) +(+1.0)* ((#1)-(+0.50))
(y) (+2771.50) (+1.0) ((#2) (+0.50))
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World Coordinate Axis Transforms for Image Block mask
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Group# Axis#
1 1,2 EQPOS(RA ) = (+246.6462)[deg] +TAN[(-0.000136667)* (sky(x)-(+4096.50))]
(DEC) (-24.3874 ) (+0.000136667) ( (y) (+4096.50))
The stack to which this observation belongs is given by the STACK_ID keyword:
unix% dmkeypar acisf00637_001N020_pixmask3.fits.gz stack_id echo+ acisfJ1626350m242314_001
ACIS Full-Field Pixel Mask
![[ACIS full-field pixel mask: the ACIS-I array is contiguous (no gaps between the separate chips) but the two ACIS-S chips have a gap between them.]](imgs/obsid637-pixmask3.png)
ACIS Full-Field Pixel Mask
An example of an ACIS full-field pixel mask for an observation (in this case ObsId 00637), where the yellow pixels indicate locations that are included in the catalog.
The pixmask3 can be compared to the fov3 file using a command similar to the following, which creates the following figure:
unix% ds9 -bin factor 4 -smooth yes \ -scale log -scale limits 0 30 -cmap invert \ acisf00637_001N020_evt3.fits.gz \ -bin filter 'energy=500:7000' \ -region acisf00637_001N020_fov3.fits.gz -mask transparency 60 -mask acisf00637_001N020_pixmask3.fits.gz \ -pan to 4600 4450
Comparison of the pixel3 and fov3 filters
![[The DS9 screen shows the event file data as a grey-scale image, overwhich the fov3 region file is shown as green polygons and the pixmask3 as red, partially transparent, rectangles. The fov3 file is slightly-larger than the actual data, whereas the pixmask3 does not cover all the data (the edges are excluded).]](imgs/obsid637-pixmask3-overlay.png)
Comparison of the pixel3 and fov3 filters
The image shows how the fov3 file is slightly-larger than the chip data (note that the pixels here have been heavily smoothed so appear to extend out further than they actually do), whereas the pixmask3 file (the pink rectangles) does not include the edges of the chips (where the exposure map changes strongly).